1. Field of the Invention
The invention generally relates to the fields of medicinal and synthetic chemistry. More specifically, the invention relates to the synthesis and use of carotenoids, including analogs, derivatives, and intermediates.
2. Description of the Relevant Art
Carotenoids are a group of natural pigments produced principally by plants, yeast, and microalgae. The family of related compounds now numbers greater than 700 described members, exclusive of Z and E isomers. At least fifty (50) carotentoids have been found in human sera or tissues. Humans and other animals cannot synthesize carotenoids de novo and must obtain them from their diet. All carotenoids share common chemical features, such as a polyisoprenoid structure, a long polyene chain forming the chromophore, and near symmetry around the central double bond. Tail-to-tail linkage of two C20 geranylgeranyl diphosphate molecules produces the parent C40 carbon skeleton. Carotenoids without oxygenated functional groups are called “carotenes”, reflecting their hydrocarbon nature; oxygenated carotenes are known as “xanthophylls.” Cyclization at one or both ends of the molecule yields 7 identified end groups (illustrative structures shown in FIG. 1).
Documented carotenoid functions in nature include light-harvesting, photoprotection, and protective and sex-related coloration in microscopic organisms, mammals, and birds, respectively. A relatively recent observation has been the protective role of carotenoids against age-related diseases in humans as part of a complex antioxidant network within cells. This role is dictated by the close relationship between the physicochemical properties of individual carotenoids and their in vivo functions in organisms. The long system of alternating double and single bonds in the central part of the molecule (delocalizing the π-orbital electrons over the entire length of the polyene chain) confers the distinctive molecular shape, chemical reactivity, and light-absorbing properties of carotenoids. Additionally, isomerism around C═C double bonds yields distinctly different molecular structures that may be isolated as separate compounds (known as Z (“cis”) and E (“trans”), or geometric, isomers). Of the more than 700 described carotenoids, an even greater number of the theoretically possible mono-Z and poly-Z isomers are sometimes encountered in nature. The presence of a Z double bond creates greater steric hindrance between nearby hydrogen atoms and/or methyl groups, so that Z isomers are generally less stable thermodynamically, and more chemically reactive, than the corresponding all-E form. The all-E configuration is an extended, linear, and rigid molecule. Z-isomers are, by contrast, not simple, linear molecules (the so-called “bent-chain” isomers). The presence of any Z in the polyene chain creates a bent-chain molecule. The tendency of Z-isomers to crystallize or aggregate is much less than all-E, and Z isomers may sometimes be more readily solubilized, absorbed, and transported in vivo than their all-E counterparts. This has important implications for enteral (e.g., oral) and parenteral (e.g., intravenous, intra-arterial, intramuscular, intraperitoneal, intracoronary, and subcutaneous) dosing in mammals.
Carotenoids, (e.g., astaxanthin), are potent direct radical scavengers and singlet oxygen quenchers and possess all the desirable qualities of such therapeutic agents for inhibition or amelioration of ischemia-reperfusion injury. Synthesis of novel carotenoid derivatives with “soft-drug” properties (i.e. active as antioxidants in the derivatized form), with physiologically relevant, cleavable linkages to pro-moieties, can generate significant levels of free carotenoids in both plasma and solid organs. In the case of non-esterified, free astaxanthin, this is a particularly useful embodiment (characteristics specific to non-esterified, free astaxanthin below):                Lipid soluble in natural form; may be modified to become more water soluble;        Molecular weight of 597 Daltons (size <600 daltons (Da) readily crosses the blood brain barrier, or BBB);        Long polyene chain characteristic of carotenoids effective in singlet oxygen quenching and lipid peroxidation chain breaking; and        No pro-vitamin A activity in mammals (eliminating concerns of hypervitaminosis A and retinoid toxicity in humans).        
The administration of antioxidants which are potent singlet oxygen quenchers and direct radical scavengers, particularly of superoxide anion, should limit hepatic fibrosis and the progression to cirrhosis by affecting the activation of hepatic stellate cells early in the fibrogenetic pathway. Reduction in the level of “Reactive Oxygen Species” (ROS) by the administration of a potent antioxidant can therefore be crucial in the prevention of the activation of both “hepatic stellate cells” (HSC) and Kupffer cells. This protective antioxidant effect appears to be spread across the range of potential therapeutic antioxidants, including water-soluble (e.g., vitamin C, glutathione, resveratrol) and lipophilic (e.g., vitamin E, β-carotene, astaxanthin) agents. Therefore, a co-antioxidant derivative strategy in which water-soluble and lipophilic agents are combined synthetically is a particularly useful embodiment. Examples of uses of carotenoid derivatives and analogs are illustrated in U.S. patent application Ser. No. 10/793,671 filed on Mar. 4, 2004, entitled “CAROTENOID ETHER ANALOGS OR DERIVATIVES FOR THE INHIBITION AND AMELIORATION OF DISEASE” to Lockwood et al. published on Jan. 13, 2005, as Publication No. US-2005-0009758 and PCT International Application Number PCT/US2003/023706 filed on Jul. 29, 2003, entitled “STRUCTURAL CAROTENOID ANALOGS FOR THE INHIBITION AND AMELIORATION OF DISEASE” to Lockwood et al. (International Publication Number WO 2004/011423 A2, published on Feb. 5, 2004) both of which are incorporated by reference as if fully set forth herein.
Problems related to the use of some carotenoids and structural carotenoid analogs or derivatives include: (1) the complex isomeric mixtures, including non-carotenoid contaminants, provided in natural and synthetic sources leading to costly increases in safety and efficacy tests required by such agencies as the FDA; (2) limited bioavailability upon administration to a subject; and (3) the differential induction of cytochrome P450 enzymes (this family of enzymes exhibits species-specific differences which must be taken into account when extrapolating animal work to human studies). Selection of the appropriate analog or derivative and isomer composition for a particular application increases the utility of carotenoid analogs or derivatives for the uses defined herein.
Synthesis of an appropriate analog or derivative and isomer composition requires a supply of starting materials (e.g., carotenoids, carotenoid synthetic intermediates). Any new synthetic route which is more efficient to a carotenoid analog or derivative and/or synthetic intermediate would be beneficial. More efficient synthetic routes would provide a more stable source of starting materials (e.g., carotenoids) which may be difficult or expensive to extract from natural sources. Synthetic routes to natural products may facilitate the synthesis of analogs and derivatives of the natural products.